The present invention relates to a capillary flow sensor and, in particular, a capillary flow sensor with a capillary bypass.
Flow control of fluids is important in many industries. For example, many processes in the manufacture of semiconductors require a precise reaction of two or more gases under controlled conditions. Mass flow meters are used to control molecular level chemical reactions.
Mass flow meters typically operate on the principle of directing fluid flow through two parallel passageways or branches. One passageway provides a sensor flow path for measuring the rate of mass flow of fluid, while the other passageway provides a main flow path through which the remainder (and bulk) of the fluid flows. The two parallel passageways are generally fluidly connected at each end, such that the total amount of fluid flowing through the flow meter is the sum of the fluid flowing in the sensor flow path plus the fluid flowing in the main flow path. In order for the mass flow sensor to provide accurate measurements, the ratio between the amount of fluid flowing in the sensor flow path and the amount of fluid flowing in the main flow path should ideally be constant over the range of flow rates, pressures, temperatures, etc. with which the flow meter is intended to operate. The actual ratio between the sensor path and the main flow path is, however, determined by the matching of the xe2x80x98pressure-flowxe2x80x99 characteristic between the sensor path and the bypass path.
Conventional mass flow meters that measure the mass flow rate of a fluid (liquid or gas) through a conduit are known. Thermal mass flow sensors that employ one or more temperature sensitive resistance elements in thermal communication with a sensor tube and provide an electrical output signal are also known. These sensors operate on the principle that the rate of heat transfer from the walls of the sensor tube, a laminar flow channel, to a fluid passing through the sensor tube is a function of the temperature gradient between the fluid and the channel walls, the specific heat of the fluid, and the mass flow rate of the fluid within the channel. Because the specific heat of a fluid does not vary greatly with pressure or temperature, a thermal mass flow sensor that is calibrated for a particular fluid will give accurate mass flow rate readings over a wide range of operating conditions.
Thermal mass flow sensors typically include one or more heating elements that transfer energy to a fluid flowing in a sensor tube usually having a cross-section of capillary size dimensions. It is common for the sensor tube to include an inlet and an outlet, each fluidly connected to a main fluid flow channel of the mass flow meter. The sensor tube inlet is typically located downstream of a main fluid flow channel inlet, and the sensor tube, outlet is typically located upstream of a main fluid flow channel outlet. As fluid flows through the sensor tube, heat is carried from the upstream heating element or resistor toward the downstream heating element or resistor, with the temperature difference being proportional to the mass flow rate of the fluid flowing through the sensor tube and the main fluid flow channel.
A bypass, or laminar flow element, is typically located in the main fluid flow channel positioned between the input and the output of the sensor tube to insure laminar flow through the bypass section of the main fluid flow channel up to a maximum designed flow rate. Above this maximum, the flow becomes turbulent. FIG. 1 illustrates a conventional flow meter 100 having a mass flow sensor tube 102 with capillary bypass 104 consisting of multiple capillary tubes 106 bundled together in parallel to provide laminar flow through the bypass. Generally, when a fluid flows through a laminar flow element, such as a bundle of capillary tubes 106, a pressure differential between opposite ends of the mass flow sensor is proportional to volume flow of fluid if the fluid forms a laminar flow. Typically, the inlet to the sensor tube is located upstream of the capillary bypass inlet, and the sensor tube outlet is located downstream of the bypass outlet. It is known that fluid flow is laminar when the Reynold""s number is not more than 2,000.
The ratio of the mass of fluid flowing though the sensor tube to the total mass of fluid flowing through the input to the main fluid flow channel is commonly referred to as the xe2x80x9csplitting ratio.xe2x80x9d The splitting ratio is determined by the geometries of the sensor tube, the main fluid flow channel, and the bypass. Ideally, the splitting ratio is constant over the entire range of mass flow rates for which the flow meter was designed. When the splitting ratio is constant, the bypass is said to be xe2x80x98linearxe2x80x99. Unfortunately, in practice the splitting ratio typically varies, both as a function of the rate of mass flow of fluid over the measurable range, and as a function of the viscosity of the fluid used for the application (e.g. the splitting ratio for one fluid may differ from that of another (e.g. more or less dense)) fluid at the same flow rate.
Sometimes the splitting ratio of a particular flow meter may be locally optimized by selecting an appropriate bypass in a conventional mass flow meter so that it is made reasonably constant within a certain range of mass flow rates. However, a conventional mass flow meter designed for low flow rate applications is not appropriate for high flow rate applications, and vice versa. On the one hand, measurement of the flow rate is accurate only if the fluid flow past the bypass is linear, and typically, flow rates at the low end of the designed flow range are more linear than flow rates at the high end of the designed flow range. On the other hand, attempts to measure too low a flow rate below the designed flow range of the bypass can result in insufficient flow rates through the sensor tube, which can lead to inaccurate measurements of the differences between the two heating elements. This may further lead to too low a sensor output signal to offer any flow readings with good accuracy. Alternatively, attempts to measure a flow rate that exceeds the bypass design flow range can render the split ratio significantly nonlinear and detrimentally affect the accuracy of the flow sensor.
Inaccurate readings may also result when a flow sensor designed for one fluid is used with a different fluid. Different fluids have different viscosities, which affects the Reynolds number (see Detailed Description below) of the flow in the tube. Since the Reynolds number governs the xe2x80x98pressure-flowxe2x80x99 characteristics, different fluids therefore produce different xe2x80x98pressure-flowxe2x80x99 characteristics in the sensor and the bypass. Also, because the sensor tube is heated, a feature of the sensor, the temperature of the fluid in the sensor tube differs from the temperature of the fluid in the main fluid or bypass channel. Because fluid viscosity varies with temperature, and pressure drop is proportional to viscosity, and different fluids have different viscosity verses temperature curves, different fluids may again have different splitting ratios in the same flow sensor.
In one embodiment, the present invention provides a flow sensor comprising a housing having a fluid inlet and fluid outlet. A bypass comprising at least one capillary tube is disposed between and fluidly connected to the inlet and the outlet, and a sensor unit is fluidly connected to the inlet and outlet via a sensor conduit. The at least one capillary tube has a length substantially equal to that of the sensor conduit. The capillary tube may also have an inside diameter and/or a cross-sectional shape substantially equal to the inside diameter and cross-sectional shape of the sensor conduit.
In another embodiment of the invention, a flow sensor comprising a housing, a bypass, and a sensor unit is provided. The housing includes an inlet and an outlet. The bypass includes at least one capillary tube disposed between and fluidly connected to the inlet and the outlet. The senor unit is fluidly connected to the housing inlet and outlet via a sensor conduit. The at least one capillary tube has an entrance effect substantially equal to an entrance effect of the sensor conduit.
Another embodiment is directed to a process for measuring fluid flow comprising passing a portion of a fluid through at least one bypass tube having an entrance effect, passing another portion of the fluid through a sensor unit having a sensor conduit with an entrance effect substantially equal to the entrance effect of the at least one bypass tube, and measuring a characteristic of the fluid in the senor conduit.
Other advantages, novel features, and objects of the invention will become apparent from the following detailed description of non-limiting embodiments of the invention when considered in conjunction with the accompanying drawings, which are schematic and which are not intended to be drawn to scale. In the figures, each identical or nearly identical component that is illustrated in various figures typically is represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. In cases where the present specification and a document incorporated by reference include conflicting disclosure, the present specification shall control.